Stimuli associated with behavioral events may induce physiological responses which serve adaptive functions for both the individual and the species. One of the primary models for study of these behavior/endocrine interrelationships has been the study of the mechanisms whereby reproductive behavior induces the neural and endocrine changes necessary for initiation and maintenance of pregnancy in the female rat. In this species, the behavioral patterns exhibited during mating are the lordosis posture and several solicitational behavior patterns which serve to indicate sexual readiness as well as to arouse the male to mate. One of the more compelling, yet least studied, aspects of solicitational behavior is the regulation, or pacing, by the estrous female of the timing of the penile intromissions received from males, behaviors occurring spontaneously under laboratory conditions in which females are placed into experimental test chambers which allow them to control their contacts with sexually active male rats. The frequency and timing of cervical-vaginal stimulation received from males during mating are critical for the cessation of ovarian cyclicity which accompanies pregnancy and pseudopregnancy (PSP); in particular, pacing of coital contacts by the female significantly increase the incidence of PSP or pregnancy above levels seen in females mated under test conditions in which they are not able to pace coital stimulation. Therefore, paced solicitation provides a behavioral mechanism whereby the female insures that she receives appropriate stimulation for pregnancy initiation. Experiments proposed in this application will examine the physiological mechanisms responsible for the display of paced solicitation and the role of the ovarian steroids, estradiol and progesterone, in the pacing of coital contacts by females, changes in the secretory pattern of prolactin (PRL) following paced as opposed to non-paced coital stimulation, and the neural sites involved in the display of pacing behavior.